I. Field of the Invention
The invention generally relates to cellular telephones and in particular to cellular telephones configured for receiving and transmitting data.
II. Description of the Related Art
Cellular telephones are becoming increasingly popular as an alternative medium for telephonic communication beyond conventional land lines such as the public switched telephone network (PSTN). Initially, cellular telephones were only provided with voice transmission capabilities for transmitting voice conversations. More recently, it has become desirable to also provide cellular telephones with the capability to receive and transmit data, such as data provided by a laptop computer or portable facsimile machine. For example, with a cellular telephone configured to receive and transmit data, a user may transmit data from a laptop computer to a base computer, perhaps at the home or office. Alternatively, the user can access the Internet using a laptop computer, perhaps to browse web pages or to access message boards. With such an arrangement, the user can also access E-mail remotely using the laptop computer and cellular telephone.
To provide data services, the data provided by the laptop computer or other data source must be converted into signals appropriate for processing and transmission by the cellular telephone. For a cellular telephone configured to process and transmit signals in accordance with the American Mobile Phone System (AMPS) protocols, the data must be converted to analog signals representative of tones of the type typically transmitted over land lines connected to the PSTN. For telephones configured in accordance with time division multiple access (TDMA) or code division multiple access (CDMA) transmission protocols, the received data must be converted to data formats appropriate for those protocols. Difficulties arise performing such conversions, particularly insofar as baud rate detection is concerned and insofar as power consumption is concerned
To efficiently process data, the baud rate at which the data is transmitted to the cellular telephone must first be determined. Without knowing the baud rate, the cellular telephone cannot effectively decode the received data for the purposes of converting the data to the appropriate format for transmission. This is particularly true for cellular telephones incorporating TDMA or CDMA. Baud rate detection should be performed in a manner such that there is no loss of data which could otherwise result in corruption of the data requiring complete re-transmission of some portion of the data. The baud rate may change dynamically during a data transmission perhaps in response to the detection of amounts of noise within the overall signal transmission system. Switching of baud rates during transmissions also occurs routinely during facsimile transmissions. Accordingly, to provide data transmission services, the cellular telephone should be capable of detecting and responding to changes in baud rate without loss of data.
One possible solution is to include a digital signal processing (DSP) chip within the cellular telephone. The DSP receives formatted signals directly from the laptop computer or other data source and determines therefrom the baud rate of the data. In the case of an AMPS cellular telephone, the DSP chip would then convert the data, in accordance with the determined baud rate, into the aforementioned tone signals for transmission to other components of the cellular telephone, such as to a microcontroller, for further processing and eventual transmission. In the case of a TDMA or CDMA cellular telephone, the DSP chip would instead convert the data, in accordance with the determined baud rate, into the appropriate data format for transmission using TDMA or CDMA transmission protocols.
Various DSP chips are available which are capable of detecting the baud rate of data and are capable of converting the data to signals appropriate for processing by the microcontroller of a cellular telephone without any loss of data. However, such DSPs are fairly expensive and consume significant amounts of power. As such, a cellular telephone incorporating such a DSP will likely be rather expensive and may have a fairly minimal power life. To minimize the cost of the cellular telephone itself and to minimize power consumption within the cellular telephone, it has been proposed to provide the DSP chip within a Personal Computer Memory Card International Association (PCMCIA) card for insertion into the laptop computer or other device. As such, the DSP draws its power from the laptop computer and not from the cellular telephone. Moreover, the cellular telephone can be sold without a DSP thereby reducing costs of the cellular telephone itself. Although such an arrangement represents an effective solution for some purposes, problems remain. In particular, to obtain a telephone capable of performing data transmission, a user must purchase not only the telephone but the separate PCMCIA card including the DSP chip resulting in a total cost which likely exceeds the cost of purchasing a cellular telephone incorporating a DSP chip. Moreover, not all laptop computers, and other data source devices, are provided with PCMCIA card slots for accommodating PCMCIA cards.
Accordingly, it would be desirable to provide an improved method and apparatus for implementing data transmission services within a cellular telephone which does not require use of a DSP chip whatsoever and which does not require interconnection via a PCMCIA card.